Our minds are constantly churning ideas for paintings, stories, plays. We see the world through the lens of our craft. Just like the time I went to Baltimore... I kept thinking, Oh! Just like The Wire! I saw the city through the lens of The Wire. Well, perhaps that was a poor comparison but maybe you get it. Anyway, that mode is always on because somewhere down the line, the craft becomes you. It takes a while to get to that point, but I think that once you get there you stop questioning and doubting yourself in the way a fearful, self-immobilizing beginner would.
I've been talking to a lot of people about discipline. I really do believe it's the deal breaker (yeah! Liz Lemon!) between hobby and professional. Time seems to be the most common excuse for why people don't pursue what they love and want. With discipline, you're intentionally setting aside the time. You're allocating the resources and putting your craft first. Discipline is a tricky skill to learn and it works differently for different people. I think it starts with intention, knowing what's important to you and rarely compromising it. It may be challenging and entering you into a whole new world of struggle, but you'll be happy in a way few, if any, can take away from you.